I. Field of the Invention
The present invention relates to a semiconductor device which has an improved organic insulating film for preventing erroneous operation of an internal element thereof, and a method for manufacturing the same.
II. Description of the Prior Art
Conventionally, an inorganic insulating film such as a silicon oxide film or an organic insulating film such as a polyimide film has been used as a passivation film of a semiconductor device, especially, as an insulating interlayer of the multi-layer wiring.
A typical structure of a conventional multi-layer semiconductor device is shown in FIG. 1. A first wiring layer 3 is formed on an insulating layer 2 (generally, silicon oxide film) on a silicon substrate 1 in which diffusion regions are formed. An insulating interlayer 4 is formed on the first wiring layer 3. After a through hole is formed in the insulating interlayer 4, a second wiring layer 5 is formed.
An inorganic insulating film formed as the insulating interlayer 4 by the CVD (chemical vapor deposition) may be rendered electrically stable by proper selection of film-forming conditions. However, formation of the inorganic film requires a relatively high temperature of 400.degree. to 600.degree. C. The inorganic film can not smooth out indentations of the underlying film. Also damage, such as cracks, can occurs due to stress (caused by e.g., the different thermal expansion coefficients) between the films.
Although an organic insulating film can be readily obtained by coating, requires only a temperature only slightly above room temperature, has the ability to smoothout the underlying film, and decreases the stress between the films, organic insulating film is not electrically stable.
The above drawbacks of an organic insulating film are based on the fact that an organic polymer such as polyimide which forms the film has a dipole moment, and that polyimide is strongly polarized upon application of an electric field. For example, in a structure such as portion A surrounded by the broken line in FIG. 1, when a positive voltage is applied across the second wiring layer 5, the organic polymer forming the insulating interlayer 4 is polarized. As a result, a base region (p-type semiconductor region) 6 of a bipolar transistor formed in the silicon substrate 1 is inverted to an n-type semiconductor region, thus preventing normal operation of the transistor. Furthermore, even after the electric field is cleared, the organic material forming the insulating interlayer 4 remains polarized. This adversely affects the element formed in the silicon substrate 1.
In a semiconductor device which uses an organic PG,5 insulating film as an insulating interlayer under the upper wiring layer, such as the second wiring layer, various limitations are imposed on the circuit design: e.g., the transistor element must not be formed immediately under the wiring layer; and any wiring layer across which a voltage is constantly applied must not be formed on the organic insulating film.